1. Field of the Invention
This invention relates to the field of linear voltage regulators, and particularly to pass transistor driver circuits for low-dropout voltage regulators.
2. Description of the Related Art
Low-dropout (LDO) voltage regulators, i.e., regulators which must operate with a small difference between their input and regulated output voltages, can be difficult to frequency compensate. U.S. Pat. No. 5,631,598 to Miranda et. al (assigned to the present assignee), described an implementation of an LDO voltage regulator which facilitated the use of a desirable frequency compensation scheme. A simplified schematic of the LDO described therein is shown in FIG. 1. A supply voltage V.sub.in is connected to the emitter 10 of a pass transistor 12, typically a pnp bipolar transistor, and an output voltage V.sub.out is taken at the transistor's collector 14. The output voltage is regulated by controlling pass transistor 12 via its base terminal 16. Regulation is accomplished with a feedback loop: a signal 17 representative of the output voltage is fed back to the non-inverting input 18 of an error amplifier 20, usually via a voltage divider 22. A reference voltage Vref is connected to the inverting input 24 of the amplifier. The amplifier's output 25 is connected to the input 26 of an inverting amplifier 28, whose output is connected to the base of a drive transistor 30 which provides the drive current for pass transistor 12. Inverting amplifier 28 and drive transistor 30 provide two inversions, and in combination form a non-inverting driver circuit 32. With a non-inverting driver circuit employed in this way, the regulator can be frequency compensated by connecting a compensation network 34 between V.sub.out and the output 25 of error amplifier 20.
However, using a non-inverting driver of this sort can present a problem, particularly when a MOS pass device is used. The large MOSFET needed to handle large output currents will have a large gate capacitance. At the same time, the pass device is driven from a high impedance node of the driver circuit--in this case, the collector of drive transistor 30. The high impedance driver combined with the large gate capacitance result in a low frequency pole in the loop transfer function, which can prove troublesome when trying to frequency stabilize the LDO in the face of highly variable load resistance and reactance.